External ion source

In anticipation of the development to operational status of the ion or direct counting systems, it would be helpful if we could compare these values with projected counting errors for the two types of direct counting systems being developed. Table 4 lists projections for the Rochester Van de Graaff facility [49] and the University of California Lawrence Berkeley cyclotron system employing an external ion source [31,50]. Table 4 also lists the sample sizes and approximate measurement periods for both systems. This data illustrates the potential extention in dating... 

The quadrupole ion trap (QIT) creates a three-dimensional RF quadrupole field to store ions within defined boundaries. Its invention goes back to 1953, [103-105] however, it took until the mid-1980s to access the full analytical potential of quad-mpole ion traps. [137-140] The first commercial quadmpole ion traps were incorporated in GC-MS benchtop instruments (Finnigan MAT ITD and ITMS). Electron ionization was effected inside the trap by admitting the GC effluent and a beam of electrons directly into the storage volume of the trap. Later, external ion sources became available, and soon a large number of ionization methods could be... 

Fig. 4.40. Schematic of a quadmpole ion trap, (a) QIT with external ion source (illustration stretched in z-direction) and (b) section in the rz-plane (in scale), (a) Reproduced from Ref. [144] by permission. lohn Whey Sons, 2000.

Fig. 4.44. Timing sequence used for mass-selective instability mode (about 1.5 cycles shown). With an external ion source the ionization time is replaced by the ion injection pulse. Reproduced from Ref. [150] by permission. Elsevier Science, 1984.

With external ion sources it became feasible to interface any ionization method to the QIT mass analyzer. [171] However, commercial QITs are chiefly offered for two fields of applications i) GC-MS systems with El and Cl, because they are either inexpensive or capable of MS/MS to improve selectivity of the analysis (Chap. 12) and ii) instruments equipped with atmospheric pressure ionization (API) methods (Chap. 11) offering higher mass range, and some 5-fold unit resolution to resolve isotopic patterns of multiply charged ions (Fig. 4.47). [149,162,172,173]... 

While some buffer gas is beneficial in case of QITs, ICR cells are preferably operated at the lowest pressure available. The typical path from an external ion source into the ICR cell is therefore characterized by multistep differential pumping to achieve some 10 -10 Pa in the cell. 

Ehlers, M. Schmidt, S. Lee, B.J. Grote-meyer, J. Design and Set-Up of an External Ion Source Coupled to a Quadrupole-lon-Trap Reflectron-Time-of-Flight Hybrid Instram t. Eur. J. Mass Spectrom. 2000, 6, 377-385. 

Mclver, R.T., Jr. Li, Y. Hunter, R.L. MALDl With an External Ion Source Fou-rier-Transform Mass Spectrometer. Rapid Commun. Mass Spectrom. 1994, 8, 237-241. 

Several other types of ion source under development should be watched closely for potential inorganic/organometallic use. Ionization by electrons from a Ni source has been used in an external ion source that is at atmospheric pressure 66,67) giving a reported sensitivity in the subpicogram range. 

For many years, filament and PIG ion sources were placed in the center of the cyclotrons. However, the development of more complicated and powerful ion sources such as the ECR sources required more space than was available at the center of the machines. Present-day cyclotrons have external ion sources and a low-energy beam from an electrostatic injector is threaded into the center via an axial channel and inflector or via a radial channel in separated- sector machines (described below). 

Pulsed-Valve Cl and CID-Exneriments. Chemical Ionization (Cl), self-CI (SCI), and direct or desorption Cl (DCI) experiments in FTMS can be done equally well with the differentially-pumped external ion source described below, or with a pulsed-valve single cell arrangement (5,6). In our experiments, we admit a pulse of reagent gas via a piezoelectric pulsed valve with a minimum opening time of about 2.5 ms (7). Unlike solenoid pulsed valves, the performance of piezoelectric pulsed valves is not disturbed by the strong magnetic field of 4.7 Tesla. 

SIMS. Secondary Ion Mass Spectrometry is particularly suited for ionization of nonvolatile, polar, and thermally labile molecules. Liquid SIMS, using liquid glycerol matrices, is best done in the differentially-pumped external ion source, because matrix effects and the high vapor pressure of glycerol make liquid SIMS unsuitable for single cell low-pressure FTMS. 

Figure 11a. Positive ions of Ultramark 1621. Electron impact ionization with direct insertion probe in external ion source.

With the successful implementation of differentially-pumped external ion sources, FTMS is rapidly becoming a routine mass spectrometric technique. Medium-pressure interfaces for the coupling of GC, LC, FAB, and liquid SIMS into the external ionizer are currently under development, and should become available in the near future. 

Recently, new 2D-methods for the analysis of complex mixtures have been developed for time-of-flight mass spectrometry (22), which could also be utilized in external ionization FTMS. Specifically, the combination of IR-laser desorption of nonvolatile neutrals, followed by adiabatic cooling to 2°K in a supersonic jet, and subsequent compound-selective Resonance-Enhanced Multiphoton Ionization (REMPI) could increase the role of FTMS in the analysis of biological mixtures. The coupling of supersonic jets to the external ion source would also be of interest in ion- and neutral cluster experiments. 

Having introduced the rudiments of ion motion in a typical FTMS ion cell, let s consider the problem of ion detection in an experiment involving (i) ion emission from a surface near the ion cell, i.e., laser desorption or Cs+ ion SIMS, and (ii) ions entering the ion cell from an external ion source. 

Ions formed in an external ion source and injected into the ICR cell are subject to effects which are analogous to the case of particle desorption ionization in the ICR cell. However, experimental results indicate limits in the resolution of high mass ions due to the finite duration of the time-domain image current signal. This observation is consistent... 

An important development for ion/molecule reaction studies by FA is the extension of the method using so-called selected ion flow tube (SIFT) facilities (Adams and Smith, 1976). In the latter configuration ions are generated in an external ion source, extracted and separated by a quadruple mass filter, after which ionic species of a single mass-to-charge ratio are injected into the flow tube. This set-up permits the ion/molecule reactions of mass selected ions to be studied in the absence of ions of other masses (similar to studies of mass selected ions in FT-ICR after application of so-called ion ejection techniques see above) and neutral precursors, while a wide choice of neutral substrates is possible. 

The first commercially available ion traps where coupled to a gas chromatograph (GC), which provided helium for ion cooling. Ionization occurred in the trap by injection of electrons through an end cap. Some GC/MS ion traps still use this internal ionization. Most instruments have an external source. Figure 2.19 shows a popular ion trap instrument with an external ion source, the Finnigan LCQ. Ions produced in the source are focused... 

A quadrupole ion trap (Figure 12.7C) mass analyzer collects ions in stable trajectories using a radiofrequency oscillating voltage on a central ring electrode. A gated electron beam ionizes neutral molecules within the trap, or ions may be injected into the trap from external ion sources. A second radiofrequency field between the end caps causes ions of a particular m/z to go into an unstable trajectory and pass through the holes in one end cap to the ion detector. Several... 

Experiments performed to date with cyclotrons have used positive ions obtained from carbon dioxide and a gas ion source. This is an advantage in the sense that it permits standard pretreatment practices developed over the last 30 years at decay counting laboratories to be routinely employed up to the point of measurement (29). On the other hand, beam currents using gas ion sources are characteristically significantly less than those from the solid samples currently used with electrostatic type accelerators. In addition, memory effects, which make comparisons of a standard to an unknown difficult, have been reported. The first cyclotrons used for radioisotope measurements had previously been used extensively for nuclear physics experiments and the production of high energy ions. Because of these experiments, some cyclotron systems have apparently been contaminated. For the 88-in. Berkeley cyclotron, the construction of an external ion source was designed to attempt to overcome this problem. Unfortunately, the efficiency of the beam transport system in the external ion source introduced other problems (30). 

An important development in quadnipole technology is the three-dimensional ion trap [38-39]. A quadmpole ion trap consists of a cylindrical ring electrode to which the quadnipole field is apphed, and two end-cap electrodes (Figure 2.3). One end-cap contains holes for the introduction of electrons or ions into the trap, while the other has holes for ions ejected out of the trap towards the electron multiplier. In LC-MS systems, ions are generated in an external ion source. The ions are introduced to the trap in a pulsed mode and stored there. A helium bath gas (0.1 Pa) is present in the trap to stabilize the ion trajectories. 

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